1. Field of the Invention
The present invention is directed to a power supply for predominantly inductive loads of the type having at least two controllable voltage sources, each of which includes a voltage-setting unit connected at its input side to a d.c. voltage source and which generates a controlled voltage output at its output terminals, and wherein the d.c. voltage sources respectively connected to the voltage-setting units are separated in potential, with the output terminals of the voltage-setting units being connected in series.
2. Description of the Prior Art
It is necessary for rapid, two-dimensional imaging in a medical nuclear magnetic resonance imaging apparatus to superimpose various magnetic gradient fields on a fundamental magnetic field in rapid succession. For example, the operational sign of a read-out gradient in such an apparatus is changed in alternation in succession in an imaging technique known as echo-planar-imaging (EPI).
One possibility for achieving such rapid changes in current, required for changing the operational sign, is to operate a resonant circuit composed of a capacitor and a gradient coil at its resonant frequency, this resonant frequency typically lying in the kilohertz range. Arbitrary pulse shapes of the read-out gradients, however, are not possible using this approach, but instead only sinusoidal pulse shapes can be produced. The echo signal which is thereby obtained must then be appropriately sampled and filtered. In addition, transient effects can deteriorate the signal which has been read out.
A power supply for feeding gradient coils of a magnetic resonance imaging apparatus, i.e., for feeding a predominantly inductive load, is described in the article "A General Purpose Non-Resonant Gradient Power System," Mueller et al., Book of Abstracts, Vol. 1, Society of Magnetic Resonance in Medicine, Tenth Annual Scientific Meeting and Exhibition, Aug. 10-16, 1991. This known power supply offers the possibility of achieving the rapid changes in current required for changing the operational sign of the gradient fields by employing a high-voltage source which is connected to the gradient coil as needed, which is arranged in parallel with a linear gradient amplifier. Although it thus possible to supply currents having a substantially rectangular waveform, with steep edges, to the gradient coil using this approach, the rise in the current is essentially unregulated, and is dependent on the ohmic resistance of the coil, its inductance, and on the voltage of the high-voltage power pack.
German OS 40 17 207 discloses a power supply of the general type initially described, which is employed in the fields of electrical discharge machining, cathode sputtering of metals, or exciting lasers for processing materials. Step-down regulators are used as the voltage-setting units, which are respectively supplied with offset clock pulses which control the operation thereof. Due to the higher ripple of the output voltage caused as a result, the inductance of the smoothing choke can be reduced compared to the value which is otherwise standard. The connected users can therefore be supplied with a rapidly varying direct current. The power supply disclosed in this reference, however, can only supply direct currents in one direction. All series-connected, controlled voltage sources, however, are active given low output voltages, i.e. only small changes in current given predominantly inductive loads, even using a lower drive (modulation) of the voltage-setting unit. For applications wherein high voltages are only briefly required, this results in correspondingly high switching losses.